The present invention relates to a keyboard with N-key rollover, and more particularly to a key output method in a keyboard, which requires no diodes for preventing a roundabout current.
FIG. 4 of the accompanying drawings is a block diagram of a keyboard input device having an N-key rollover function. A key matrix 1 is composed of a matrix of keys 2 and diodes 3 connected in series thereto, respectively. The key matrix 1 has columns 1a connected to a decoder 4. The decoder 4 and lines 1b of the key matrix 1 are connected to a CPU 5.
A ROM 6 storing key codes and a program for issuing them is connected to the CPU 5.
When one of the keys is depressed, a scanning signal from the decoder 4 is applied through one of the columns 1a, the depressed key and the diode connected thereto, and a corresponding one of the lines 1b, to the CPU 5. The CPU 5 detects the depressed key based on the scanning position (column position) and the line position. A key code corresponding to the detected key is then read out of the ROM 6 and issued.
While the keyboard is operated on at a high speed, a plurality of keys are sometimes depressed concurrently. The N-key rollover function allows all output signals from the depressed keys to be issued. This rollover function requires the use of diodes 3 associated respectively with the keys 2 for the reason that when three or more keys sharing common rows and columns are concurrently depressed, a roundabout current flows to let an undepressed key be detected as being a turned-on key. This condition is known conventionally as "a phantom switch condition", since depression of three keys at three corners of a rectangle of rows and columns causes erroneous detection of the fourth corner as being depressed.
The keyboard with the N-key rollover function is therefore of a high cost since as many diodes are required as there are keys.